JPH0642990B2 - Plasma arc heating or surface treatment method by magnetic drive - Google Patents

Description

TECHNICAL FIELD The present invention relates to a plasma arc treatment method for efficiently performing heat treatment or surface modification of a metal plate, for example.

[Prior art]

Plasma arcs or arcs (hereinafter simply referred to as arcs) are due to the pinch force due to the electromagnetic interaction between the magnetic field created by the arc current and the arc current itself, or due to the action called thermal pinch due to the ambient cooling effect due to the gas flow. Therefore, the arc contracts and the energy density is high. This pinched high energy density state is suitable for the purpose of heating or melting a narrow region at high speed, and has been widely used for welding and cutting conventionally. It is also well known that the gas is chemically activated in the high temperature plasma state, and it has been used for the reaction treatment of the surface of a narrow area.

However, in some cases, heating, melting, or surface treatment may have to be performed over a wide range.
The narrowed arc is not suitable for such a purpose, and a method of controlling the arc shape with a magnetic field to widen the width has been considered.
There are two methods for controlling the arc shape. The first is a method of applying a steady magnetic field such as a cusp type so that the arc spreads along the lines of magnetic force. Since this method requires a strong magnetic field, it is not widely adopted in reality. The second method is to apply an alternating magnetic field to the arc orthogonally, and use electromagnetic force that acts on the arc to oscillate the arc so that it reciprocates. is there. A method for generating such an oscillating arc is described in Japanese Patent Publication No. 58-047273. In the second method, a sine wave having a commercial frequency of 50 Hz or 60 Hz has been exclusively used as a current flowing in the exciting coil in order to generate an alternating magnetic field. The heating atmosphere has been implemented at 1 atm.

[Problems to be Solved by the Invention]

In the conventional arc oscillating method that uses an exciting current that changes sinusoidally with time at a commercial frequency, the material to be processed does not move a distance greater than the thickness of the arc within one cycle of the arc oscillating motion. Then, the entire area of the material surface within the oscillation amplitude can be heated thoroughly. However, if the material to be processed moves within a period of one cycle of the arc oscillating motion over a distance equal to or greater than the thickness of the arc, the arc point leaves a sinusoidal locus on the surface of the material to be processed, and only on this locus. However, there is a problem that heating is not uniform because the entire width of the oscillation amplitude of the arc is not heated without a gap. Further, even if the oscillating arc is used to broadly heat, it may be important to control the heat input distribution in the width direction. There is one problem. When the material to be heated is used as an electrode and the arc point is oscillated on the surface of this material by the action of electromagnetic force, if this is carried out under normal pressure, the movement of the arc point does not always go smoothly, but for a short time. The arc point may be fixed at a certain point on the surface of the material to be heated. Such discontinuous swinging behavior of the arc point leaves a small melting mark on the surface of the material, leading to surface defects. This is the third problem of the conventional method.

Because of the above-mentioned serious problems, the magnetically driven oscillating arc has not been widely used, although it is a characteristic heat source having a high energy density and a wide width. The present invention is to solve the above problems.

[Means for Solving the Problems]

The present invention relates to a method of heating or surface-treating using a plasma arc that is oscillated by magnetic drive, in which the oscillation frequency is controlled, the oscillation waveform is controlled, or the pressure of a heating atmosphere is controlled, either alone or in any one of them. The present invention relates to a plasma arc heating or surface treatment method characterized by performing two or all of them in combination.

The swing frequency and swing waveform are changed by changing the frequency and current waveform of the current connected to the exciting coil. The pressure of the heating or surface treatment atmosphere is controlled by being performed in a container equipped with an exhaust device.

The present invention relates to a surface treatment method involving heating, dissolution, or reaction, but the heat treatment will be mainly described below as an example, except where it is particularly necessary to explain. Therefore, even if only heating is mentioned, its meaning includes surface treatment involving dissolution or reaction.

[Action]

In the present invention, the oscillation frequency is controlled in order that the material to be processed which moves relatively at high speed with respect to the arc is heated thoroughly. The diameter of the plasma arc is Dcm, the oscillation frequency of the arc is Hz, and the material to be heated has a relative velocity Vcm / sec perpendicular to the oscillation surface of the arc.
In the case of moving at, if the moving speed is high and the swing frequency is small, the entire arc swing width cannot be heated. In order to heat the entire swing width uniformly, at least the arc must end the one-cycle oscillatory motion when the material to be processed moves D cm in the direction perpendicular to the arc swing surface. The condition that the oscillation period must be satisfied because it is heated thoroughly is determined by the arc thickness and the relative moving speed of the material to be treated, and is given by the following equation.

> V / D (1) Usually, the diameter D of the plasma arc is about 1 cm. Therefore, when the relative speed V is higher than 100 cm / sec which is usually practical, the arc is at a commercial frequency as in the past. If the rocking is performed, an unheated portion will be generated. When the processing speed is high, by adopting a high frequency higher than the commercial frequency, preferably a frequency of 100 Hz or more so as to satisfy the expression (1), the heat treatment can be performed over the entire swing width without any gap.

Further, in the present invention, the rocking waveform is controlled to control the heat input distribution to the material to be processed in the rocking width direction. When heating or melting the material to be treated, the uniformity of heating or the temperature distribution suitable for the purpose is often a problem. Achieving this purpose cannot be achieved only by controlling the oscillation frequency and amplitude. The temperature distribution on the surface of the treatment material is determined by the heat input distribution, and the heat input is determined by the product of the energy density of the arc and the arc residence time. Therefore, in order to control the heat input distribution in the width direction, the arc current may be appropriately controlled in time, or the swing speed of the arc may be changed depending on the position in the width direction.

Further, according to the present invention, the swing speed of the arc is controlled by changing the waveform of the exciting current for swinging the arc. For example, in order to make the heat input distribution uniform, the exciting current may be made closer to a triangular wave than a conventional sine wave. If there is a large amount of heat loss at both ends of the amplitude and the heat input at both ends is large, and it is difficult to obtain a uniform temperature distribution, it is sufficient to excite the sine wave with a current waveform that is flattened by cutting the upper and lower sides.

In the present invention, the pressure in the atmosphere is controlled in order to prevent the composition change of the surface of the material to be treated or the occurrence of surface defects when the heat treatment is performed by the swinging arc. In order to improve heating efficiency, the material to be treated is often used as one electrode of the arc. When the arc is oscillated in one atmospheric pressure, even if a fluctuating magnetic field is applied, the arc point may stay at one point for a short time, and the arc point may not move smoothly. When this phenomenon occurs, a small local melting point is formed on the surface, which causes surface defects. Experimental results have shown that the occurrence of this surface flaw can be prevented by performing the swing arc treatment in a reduced pressure atmosphere. The occurrence of local melting points decreases as the atmosphere is depressurized. It is desirable to maintain the processing atmosphere at a pressure of 300 Torr or less so that the generation of melting marks can be substantially ignored. By using a reduced pressure atmosphere, surface oxidation can be prevented, and a special reaction such as a reduction reaction, a nitriding reaction, or a carburizing reaction can be simultaneously performed by controlling the atmosphere composition.

〔Example〕

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a conceptual diagram of an experimental apparatus used for carrying out the method of the present invention. The plasma arcs 6 and 7 are the plasma torch 1,
It is generated between the heat-treated metal body 2 which is also an anode.
The plasma torch 1 and the heat-treated metal body 2 are connected to a DC plasma power supply 3. An exciting coil 4 for generating an alternating magnetic field acting on the plasma arcs 6 and 7 is installed with the plasma arcs 6 and 7 sandwiched between them, and a current source 5 capable of varying the frequency and changing the waveform shape is connected to the plasma arcs 6 and 7. A magnetic field having a component orthogonal to the arcs 6 and 7 can be generated. When an alternating magnetic field is excited and acts on the arc, the arc deforms. The direction of the magnetic field causes the deformation to be reversed, so that the arc oscillates between 6 and 7 depending on the temporal change of the magnetic field strength. The heat-treated metal body 2 can be moved back and forth by a drive device 8 having a variable speed. The plasma torch 1, the exciting coil 4, and the heat-treated metal body 2 are housed in a container 9 for controlling the atmosphere, and the container 9 for controlling the atmosphere is provided in a vacuum exhaust device 10 and a plasma torch 1.
Are connected to various gas cylinders 11, respectively.

Table 1 is an example showing the relationship between the oscillation frequency of the plasma arc and the surface heating area ratio of the heat-treated metal body.
A stainless steel plate is used as the heat-treated metal body, and the stainless steel plate is 1 m at right angles to the oscillating surface of the plasma arc.
It is moving at a speed of / sec. The oscillation frequency of the plasma arc changes the frequency of the current supplied from the excitation power source, and it is 200 Hz in the case of commercial 50 Hz which has been used conventionally.
Compared with the case of high frequency. The effect of frequency change is observed by observing the surface of the steel sheet after the heat treatment and looking at the area ratio between the heat-affected zone and the non-heat-affected zone. In the case of 50 Hz oscillation, about 50% of the area not affected by heat is observed, while
In case of 200Hz oscillation, almost 100% heat affected zone appears. From this fact, in order to perform surface heating uniformly over the entire swing width, it is necessary to select a swing frequency according to the moving speed of the heat-treated metal body, especially at a high frequency when moving at high speed. It was confirmed that rocking was necessary.

FIG. 2 shows the effect when the current waveform of the exciting current for plasma arc oscillation is changed. While moving the stainless steel plate at 0.5 m / min, rocking plasma arc irradiation heating is performed to melt the surface thinly. After the heating and melting treatment, the plate was cut and the structure was observed to estimate the melting state at the time of irradiation. FIG. 2 (a) shows the surface remelting and solidifying shape when it is oscillated with a sinusoidal waveform exciting current at a commercial frequency, and FIG. The surface remelting solidification shape when moved is shown. Due to the arc swing due to such an exciting current, the arc stays longer at both ends of the vibration amplitude, and the heat input to both ends increases, and as a result, the amount of heat loss at the ends is compensated for. It has a uniform penetration depth up to the part.
From this result, it was confirmed that the heat input distribution or the surface temperature distribution can be controlled by changing the waveform of the exciting current.

Even if the oscillating frequency and the exciting current for oscillating are optimized, if heating is performed at 1 atm, surface defects occur on the surface of the non-heated body. This defect can be prevented from occurring by lowering the atmospheric pressure. This example is shown below. A stainless steel plate was used as the non-heat-treated metal body, and this was used as the oscillating arc anode. 0.5 m / s in the direction perpendicular to the oscillating arc surface.
It was moved at a speed of ec and the surface pressure was observed by changing the atmospheric pressure. In 1 atmosphere of argon, small melting marks with a diameter of about 1 mm were scattered on the surface of the stainless steel plate after the heat treatment. On the other hand, when the heat treatment was performed in an argon atmosphere of 200 Torr, no melting mark was observed on the surface. From this, the effect of suppressing the generation of surface defects due to reduced pressure was confirmed.

In each of the examples of the present invention, the object to be heat-treated is a metal, and this is operated as an anode electrode. However, it can also be applied to the generation of non-transferred plasma jets.

〔The invention's effect〕

According to the heating or surface treatment method based on the above-described method by the oscillating plasma arc, the surface of the metal to be heat-treated which moves at a relative velocity with respect to the oscillating plasma arc is distributed over the entire oscillation amplitude, and It becomes possible to perform treatment while giving an arbitrary heat input distribution, and strict control of heating or surface treatment becomes possible. For this reason, it is possible to perform heat treatment and surface treatment with extremely high spatial homogeneity, greatly improve the reliability of the treatment material, and simplify the inspection process after the treatment.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional conceptual view showing an example of an apparatus for carrying out the present invention, and FIG. 2 shows an exciting current waveform and a shape of a melting and resolidifying portion of a conventional method and the present invention method, and FIG. This is the shape of the melting and re-solidifying portion obtained when oscillating by a conventional sinusoidal excitation current, and FIG. 6B shows the shape of the melting and re-solidifying portion when the upper and lower parts of the sine waveform of the present invention are oscillated by the excitation current. The shape of a solidification part is shown. 1 ... Plasma torch, 2 ... Heat-treated metal body, 3 ... DC plasma power supply, 4 ... Excitation coil, 5 ... Excitation current power supply, 6 and 7 ... Plasma arc deformed by magnetic field, 8 ... … Heat-treated metal moving device, 9… Atmosphere control container, 10… Vacuum exhaust device, 11… Gas cylinder, 12… Sinusoidal waveform excitation current, 13… Controlled excitation current waveform, 14… … Conventional method melt resolidification part, 15 …… Invention method melt resolidification part.

Claims (3)

[Claims] 1. A heating or surface treatment method using an oscillating arc by applying an alternating external magnetic field, comprising:
A plasma arc heating or surface treatment method, wherein the treatment is performed in a reduced pressure atmosphere of 0 Torr or less. 2. The heat input and reaction distribution are controlled by changing the excitation current waveform.
The method for plasma arc heating or surface treatment according to. 3. When the diameter of the plasma arc is Dcm and the relative velocity of the material to be processed perpendicular to the arc oscillating surface is V / sec, the exciting current frequency Hz is alternating using a current having a frequency higher than V / D. The plasma arc heating or surface treatment method according to claim 1 or 2, wherein the arc is oscillated by a magnetic field.